FIG. 1 is a diagrammatic view of a conventional digital system 100 including a transmitting device 110 and a receiving device 120. The transmitting device 110 includes a driver circuit 112 for outputting a signal generated by the transmitting device 110 onto a signal line 115. In this figure, the signal line 115 contains a single conductor for transmitting a single-ended signal, i.e., a signal representing the voltage difference between the conductor and a ground voltage. Alternatively, the signal line 115 may contain two conductors for transmitting a differential signal, i.e., a signal representing the voltage difference between the two conductors. The receiving device 120 includes a receiver circuit 122 for receiving the signal transmitted across the signal line 115.
The transmitting device 110 and the receiving device 120 are integrated circuits that may have incompatible DC I/O signal levels such that the signals generated by the transmitting device may not be correctly received by the receiving device. Specifically, the transmitting and receiving devices may operate using different offset voltages (i.e., the voltage of the DC component of a signal). To enable such incompatible transmitting and receiving devices to communicate with each other, an interface circuit 130 is connected to the signal line 115 between the devices. The interface circuit 130 removes the offset voltage from the signal output by the transmitting device 110 and biases the signal with a new offset voltage appropriate for the receiving device 120. The interface circuit 130 removes the offset voltage from the signal using either inductive or capacitive AC coupling. An interface circuit using inductive AC coupling includes relatively expensive inductor components. By contrast, an interface circuit using capacitive AC coupling includes capacitor components, which are less expensive than inductor components. However, an interface circuit using capacitive AC coupling is susceptible to data transmission errors caused by electrical noise. Although the electrical noise can be suppressed by shielding the signal line 115, the shielding increases the cost of the system.
In view of the shortcomings of prior art interface circuits, it is an object of the present invention to provide an apparatus and method for interfacing integrated circuits having incompatible I/O signal offset voltages that is relatively inexpensive to implement and yet is also insensitive to noise.